1. Field of the Invention
This invention relates to a method of fluidized catalytic cracking. More particularly, it relates to an improved method for recovery of hydrocarbon vapors from the reactive mixture of hydrocarbonaceous material and catalyst in a separator after such mixture is discharged from a riser reactor.
The object of this invention is to reduce both secondary cracking of valuable liquid products and reduce the solids content of recovered hydrocarbon products by rapid and efficient separation of cracking catalyst from product hydrocarbon vapors. This separation of catalyst solids from hydrocarbon vapors is made more efficient by construction of a special zone between the exit of the reactor tube and the entrance of the cyclone separators. This zone is oriented relative to the reactor tube in such a way that all vapors from the reactor tube travel through it but surprisingly few catalyst particles can enter. This special zone rapidly separates the hydrocarbon products from nearly all catalyst particles upon leaving the reaction zone and reduces the catalyst loading on the cyclone separators which further separate catalyst from product vapors in the conventional manner. The special separation zone is positioned so that the vapor must turn through an acute angle after exit from the riser. Preferably, for minimal secondary catalytic cracking, due to excessive time at elevated temperatures in the presence of catalyst, or minimal solids in the recovered vapor due to catalyst entrainment, or both, the inlet to a separate vapor recovery zone is located so that vapor flow to the cyclone intake is adjacent to and substantially countercurrent to the direction of flow of the reaction mixture discharge into the separation vessel. More preferably, the vapor recovery zone is formed as a low volume shroud surrounding the riser reactor pipe and extends through the upper center portion of the vessel, with the intake to the cyclones enclosed in the vessel connected to the upper end of the shroud. In this way catalyst in the hydrocarbon vapor-catalyst mixture entering the vessel is carried by inertia (and if downwardly, also by gravity) in the same direction as the conduit discharge. In accordance with the present invention, rather than directing the flowing mixture directly into cyclone separators, or disposing the cyclone inlets at substantially 90.degree. to the reactor discharge in the separator vessel, vapor that is least contaminated by catalyst fines is shown herein to be adjacent to the riser reactor outlet. Such vapor is vented through the vapor recovery shroud to the cyclones so that it is promptly delivered to a distillation column. The entry to such a vapor recovery shroud is placed within the vessel adjacent to the riser reactor discharge or outlet so that vapor must turn through an angle substantially greater than 90.degree. to enter the zone, and preferably at an acute angle of at least 180.degree..+-.30.degree., and most preferably, 180.degree. to the direction of flow from the riser reactor outlet.
2. Description of the Prior Art
Fluidized cracking of heavy petroleum fractions is one of the major refining methods to convert crude petroleum oil to useful products such as fuels for internal combustion engines. In such fluidized catalytic cracking, (known popularly as "FCC") high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided solid catalyst particles in an elongated riser or transfer line reactor. The reactor is usually in the form of a riser tube and the contact time of the material is on the order of a few seconds, say one to ten seconds, and generally not over about five seconds. This short contact time is necessary to optimize generation of gasoline and middle distillate fractions. By proper selection of temperatures and reaction times the catalytic cracking reaction is "quenched" so that economically undesirable end products of such a reaction, light gases and coke, are held to a minimum and the yield of desired products, gasoline and middle distillate oils, is at a maximum. During this short reaction period, catalyst at temperatures in the range of about 1100.degree. F. to 1450.degree. F. contacts a hydrocarbon feed stock, frequently in the form of vacuum gas oil, cycle oil or the like, heated initially to a temperature of from about 300.degree. F. to 750.degree. F. Generally the hot catalyst and hydrocarbonaceous material mixture is fluidized by steam and the reacting hydrocarbon gases. Reaction of the mixture creates large volumes of gaseous hydrocarbons by vaporization of the oils due to exposure to hot catalyst and by cracking the hot hydrocarbons to lighter gaseous hydrocarbons. The hydrocarbon vapors and catalyst mixture flow out of the riser tube into a separator or disengaging vessel. The spent catalyst is separated, primarily by gravity, and inertial forces acting on the catalyst, in the separator vessel and pass downwardly through a stripper section for return to a regenerator. Fluidizing steam also flows up through the down-flowing catalyst to assist in stripping hydrocarbon vapor from the spent catalyst. Heat for the process is added to the system by burning the coke, primarily carbon, on the spent catalyst by oxygen flowing into the regenerator. The regenerated and heated catalyst is then recirculated to the riser reactor. The desired product, hydrocarbon vapor, is recovered overhead from the separator vessel. Generally, this recovery is through one or more cyclone separators within the separator vessel connected to a plenum chamber or common piping and directly piped to a distillation column. Vapor flow through the cyclone separators extracts entrained catalyst fines. The catalyst fines recovered by the cyclone separators are delivered to the stripping section through "dip legs" connected to the lower part of the disengaging vessel in which the stripper is located. The hydrocarbon vapors from the cyclones are recovered overhead, either through a plenum chamber or through additional stages of cyclones, or through direct piping to the product fractionator.
A particular problem in the recovery of the vapor is that in spite of the separating action of the cyclones, even several stages, catalyst "fines" tend to flow with the vapor particularly at high inlet velocities and thus get transported with the hydrocarbon vapors to the distillation column. Further, prolonged contact of the hydrocarbonaceous vapors with catalyst results in secondary cracking of the desired gasoline and middle distillate fractions generated in the initial catalytic reaction. Such secondary cracking creates additional gases which are less valuable than the middle distillate oils such as gasoline from which the additional gases are made. Catalyst fines additionally degrade the recovered distillate product, and create a recovery or disposal problem. Further, high levels of catalyst entering the cyclones promote secondary cracking and fines carryover. Since the catalyst particles normally include precious materials such as zeolite crystals containing rare earth metals and sometimes platinum or palladium, the cost of replacement or recovery of these materials may be substantial. Catalyst loss in the overhead requires make-up catalyst to be added (at significant cost) to the fluidized catalytic cracking system. Any catalytic material recovered in the fractionator bottoms lowers the economic value of that oil. If economically feasible the fractionator bottoms are returned to the cracker (at the expense of yield selectivity loss) or the catalyst is allowed to settle out in an expensive decant oil separator vessel.
It is known, as noted, that the output from a riser reactor may be directly discharged into a cyclone separator system. U.S. Pat. No. 3,785,782, Cartmell, illustrates one system in which a riser discharges the entire mixture directly into a first stage cyclone and the vapor passes through a second stage cyclone prior to recovery.
U.S. Pat. No. 4,066,533, Myers, et al, discloses a system in which a riser pipe enters a separator vessel with the catalyst mixture being discharged vertically upward against the end wall of a separation vessel. Vapors are recovered from near the end of the reactor pipe directly into a first and second stage cyclone arrangement for recovery of the vapor. U.S. Pat. Nos. 4,295,961 and 4,310,489, both issued to Fahrig, et al, disclose systems in which the riser pipe enters the separator vessel and upward flow is diverted downwardly by a discharge shroud. Vapor is withdrawn from the separator vessel through cyclone separator inlet openings at 90.degree. to the shroud.
In other systems generally used in fluidized catalytic cracking, the cyclone separator intake is simply positioned in an upper part of the separator vessel. The catalyst and hydrocarbon vapor mixture is discharged, either up or down, by the reactor riser into the separator. Where the riser enters the center of the vessel, most frequently the cyclones are positioned around the outside of the vessel so that the diplegs return catalyst fines by gravity to the stripper through the annular area around the riser pipe. U.S. Pat. No. 3,957,443, Strickland, et al, and U.S. Pat. No. 2,514,288, Nicholson, are illustrative of this arrangement. In Nicholson, reactor discharge is directed upwardly into the separator vessel. In Strickland, flow is downwardly through a shroud around the riser pipe. However, a valve mechanism permits the catalyst and hydrocarbon mixture to be discharged directly upward toward the top of the vessel by a valve means. U.S. Pat. No. 3,841,843, Williams, et al, discloses a similar arrangement for flow reversal of the discharge from the reactor pipe into the separator vessel and to direct the catalyst downwardly toward the stripper zone. U.S. Pat. No. 3,243,265--Annesser, assigned to the assignee of the present invention, discloses a concentric discharge shroud around the riser pipe to discharge the resultant mixture of hydrocarbonaceous material and catalyst downwardly into a large diameter separation vessel.
U.S. Pat. No. 3,785,962, Conner, discloses a system in which the riser pipe discharge includes a 90.degree. turn so that mixture flow is essentially downward, but the vapor inlet to the cyclones is directly adjacent to the riser discharge so that in effect vapor and any entrained particles must turn 90.degree. to pass from the reactor discharge into the vapor recovery cyclones.
U.S. Pat. No. 3,826,738--Zenz discloses a riser reactor pipe which includes an outer concentric pipe section for direct discharge of the hydrocarbon-catalyst mixture downwardly into a vessel having an enlarged diameter stripping section. The vessel is formed with a small diameter section for vapor recovery prior to cyclone separation that is concentric to the discharge section of the riser pipe. Due to catalyst discharge into the stripper section, there is little room for catalyst to separate from the product hydrocarbon vapors. Further, the velocity of the vapors rising from the stripping section is increased by the restricted volume in the vapor recovery section so that rather than decelerating upon release to a large diameter vessel after discharge, the vapor velocity is accelerated which prevents disengagement of catalyst rising with vapors from the stripping section. More importantly, the cyclones cannot be accommodated within the restricted volume of the vessel so they must be positioned outside. This creates a safety hazard because hot combustible vapors, such as gasoline, propane, butane and light oils, must be well-contained to avoid the risks of fire and explosion in the atmosphere.
Such problems arise because the cyclones are highly susceptible to erosion and abrasion by the flow of catalyst with the product vapors. Such erosion increases as the cube of the velocity (V.sup.3) and the highest velocity of an FCC system is in the cyclones. Practical and safe designs for commercial installation do not have high erosion components such as cyclones containing hot combustible gases on the exterior of the vessel. Exterior walls that confine combustible gases must contain low velocity flows to minimize leakage caused by erosion.